Single-pass tillage implements providing both shallow and primary tillage in a single pass using discs and chisel points are disclosed in U.S. Pat. Nos. 4,245,706; 4,403,662; and 4,538,689. These machines, however, leave furrows and/or ridges in the soil after use. It has become desirable to leave the surface of the soil as level as possible after tilling in the Fall to reduce the amount of soil preparation necessary before planting the following Spring. U.S. Pat. No. 5,080,178 discloses a disc leveler for use with a tillage machine such as is disclosed in U.S. Pat. No. 4,403,662.
During the late 1970's, before the machine shown in U.S. Pat. No. 4,403,662 became available, primary tillage was accomplished principally with moldboard plows. The moldboard plow leaves large slabs and chunks of soil that tend to break down over the freeze and thaw cycles of Winter, but this tillage practice required substantial working of the soil in the Spring in order to level the field and prepare it for planting of the next crop. Moreover, moldboard plowing is not an effective remedy for soil erosion, and has a tendency to exacerbate erosion.
In recent years, farmers have been looking for ways to decrease soil and wind erosion. The use of a large disc assembly in front of a chisel plow on a parabolic shank accomplished these goals and also breaks up the hard plow pan (or "sole") that is created at the particular depth at which the plow is set to operate, caused by repeated tillage at the same depth over the years. The parabolic shank and winged point of recent improved plows have reduced soil erosion, but this practice also may create large chunks of soil, and it usually requires substantial spring soil-working to prepare an adequate seedbed for planting.
During the 1970's, the cutter chisel was widely used. It consisted of a chisel plow with a row of coulters to cut the residue ahead of two rows of staggered shanks on thirty-inch centers. These shanks had a four-inch twisted point attached to them to perform the primary tillage. The tip of the point was at approximately a 45.degree. angle to the horizontal, sloping downwardly and forwardly from the shank. The worked soil followed the curvature of the "C"-shaped shank that was attached to the chisel plow and was twisted in order to provide a moldboard-type turning action. A "C"-shaped shank, of the type described, is shown in U.S. Pat. No. 4,403,662.
For early cutter chisel plows, there was a net lateral movement of soil. A machine with, for example, 11 shanks would be equipped with five right-hand and six left-hand twisted points. The result was that a wide groove and a large berm were left after a pass. The machine as shown in U.S. Pat. No. 4,403,662 was an improvement because it left a smaller groove and not as large a berm by using fore and aft sets of discs and an improved point.
In U.S. Pat. No. 4,538,689, there is disclosed a winged point mounted on a parabolic shank. That winged point, in the combination shown, creates a large, rough surface similar to the surface of the moldboard plow used during the late 1970's and early 1980's. The wings on these points are set at a soil lift angle of approximately 30.degree.. This lift angle was conventional at the time, but it is an aggressive angle which causes the wings to lift the soil abruptly. In some soils, particularly more compacted soils, the combination of an aggressive lift angle on the wings of the point, together with a parabolic shank, which is designed to lift and heave soil, lifted larger soil chunks and threw them out of the paths of the chisel plow and away from cooperating discs, making it difficult to create a level soil surface after a pass of the machine.
During the 1980's, farmers desired less tillage to prepare for planting. The furrows left by the chisel shanks had to be filled with the berms that were created between each shank. In order to fill these furrows behind large parabolic shanks, smaller shanks were placed to run shallower and were placed midway between the large chisel shanks. This resulted in smaller grooves on reduced centers. With the development of the disc leveler shown in U.S. Pat. No. 5,080,178, the furrows behind the shanks were substantially filled without leaving substantial grooves after the shanks had passed, thus improving the levelness or "smoothness" of the surface.
In heavily compacted soils, a parabolic shank lifts out large chunks of soil and heaves them to the path of least resistance around the shank. These large chunks are difficult to rearrange in such a way as to give a smooth resulting surface. If a chunk is rather large, it will tend to ride up the shank, roll forward and not be in the proper location for the leveler to roll it back to its original location, because the leveler primarily moves the soil laterally, not backward.